The present invention relates to a method and apparatus for making steel in an electric arc furnace ("EAF") equipped with an auxiliary heat source including means for introducing auxiliary fuel and oxidizing gas into the furnace for the purpose of reducing the consumption of electrical energy and increasing furnace throughput rate.
More particularly, it relates to a method of making steel in an electric arc furnace equipped with at least one burner that comprises a means for injection of solid carbonaceous fuel and/or oxygen.
Previously known methods of electric steelmaking include multiple movable or permanently fixed burners utilizing hydrocarbon fuel such as, for example, natural gas or oil, at least one movable oxygen lance for injection of a stream of oxygen toward the molten bath for refining purposes and a movable means for injecting solid carbonaceous fuel for combustion and slag foaming purposes.
When an electric arc furnace operates without burners, the charged scrap is rapidly melted at the hot spots at regions of highest electric current density. This creates harsh conditions for the water cooled furnace wall and refractory lining located at the hot spots due to excessive exposure to heat from the arc during the last part of the melt down cycle. Scrap located in the cold spots receives heat from the arc at a reduced rate during the melt down cycle, thereby continuing to protect the water cooled panels and the part of the refractory lining located at cold spots of the electric arc furnace from excessive exposure to heat at the end of the melt down cycle. This asymmetrical heat distribution from the arc and non-uniform wear of the furnace walls are typical for both alternating current and direct current arc furnaces operating without burners.
Presently known burners for electric arc steelmaking use either oxygen or a combination of oxygen and air to oxidize hydrocarbon fuel. These burners are preferably installed at the colder spots of the furnace to primarily provide auxiliary heat during scrap melting in order to make the melting pattern more uniform.
Cold spots are typically formed in areas further away from the furnace arc as scrap located in these areas receives electrical energy at a reduced rate per ton of scrap. A typical example of such a cold spot is the tapping spout, due to its location away from the arc. Another cold spot occurs at the slag door due to excessive heat losses to ambient air infiltrated through this area. It is common for furnaces utilizing additional injection of materials, such as slag forming material, direct reduced iron, etc., (which is carded out through the slag door or through an opening in the furnace side wall) to create cold spots due to localized charging of additional heat consuming materials during the melt down cycle. Thus, a portion of the working volume of the furnace at the cold spots is continuously occupied with build-ups that are melted only at the end of the melt down cycle or that remain unmelted at the end of the melt down cycle when the furnace has reached its highest temperature. These build-ups reduce the working volume available for scrap to be charged and, therefore, reduce furnace throughput capacity.
Electric arc furnaces equipped with burners located at cold spots provide improved uniformity of scrap melting and reduce build-ups of materials at the cold spots. When auxiliary heat sources such as burners are placed in the electric arc furnace, their location is chosen to avoid further overheating of hot spots resulting from the rapid melting of scrap located between the electrode and the furnace shell. More specifically, the burners are located as far away from hot spots as is practically possible and the burner flame outlet opening direction is chosen so that flame penetration occurs predominantly into the scrap pile located at the cold spots.
The same philosophy is used to select the location of other additional auxiliary heat sources including oxygen injection lances. When additional lances are located at the cold spot(s), the exothermic energy of melt refining can be used more effectively to melt the scrap without overheating of the hot spots.
Injection of oxygen for melt decarburization is accomplished by one or more movable devices such as submerged, consumable oxygen pipes and/or by one or more water-cooled non-submerged oxygen lances. During operation of the water-cooled lance, the lance is first introduced into the furnace, then gradually moved to the position in which the lance discharge opening or openings for the introduction of oxygen are located, preferably approximately 150-300 mm or more above the bath. The discharge velocity of the oxygen stream from the water-cooled lance is to be chosen to allow the stream of oxygen introduced by the lance located in the above working position to penetrate the slag and to react with the iron-carbon melt without molten metal splashing on the furnace walls and electrode(s).
The slag door is the largest opening commonly used for introduction of additional chemical energy of fuels into the furnace via burner means and carbon injection means. Unfortunately, opening the slag door results in a substantial infiltration of cold ambient air into the furnace, and the slag door is typically located further away from the electrode(s) than the furnace shell. This ambient air infiltration at a distance from the electrode(s) results in a lengthening of the time needed to melt scrap at the slag door when no auxiliary heat source is operated at the cold spot near the slag door.
Combined injection of carbon and oxygen via the dedicated lances through the slag door has become a common practice for adding extra heat to the process. An additional heat source is created by the oxidation of injected carbon with injected oxygen near the cold spot at the slag door. Carbon and oxygen are typically injected by a door lance using a lance manipulator to position oxygen and carbon injection lances through the slag door by remote control. These oxygen and carbon injection lances are usually held by a common carrying arm, so that their position is fixed relative to each other during manipulation. The supply of controllable carbon flow for injection is obtained from a carbonaceous material dispenser by a compressed gaseous carrier such as compressed air, natural gas, nitrogen, etc.
The use of the burners together with carbon and oxygen lances has allowed electric steelmakers to substantially reduce electrical energy consumption and to increase furnace production rate due to the additional heat input generated by the oxidation of carbon, and by significant increases in electric are thermal efficiency achieved by the formation of a foamy slag layer that insulates the electric arc from heat losses. The foamy slag also stabilizes the electric arc and therefore allows for a higher electrical power input rate. The foamy slag layer is created by CO bubbles which are formed by the oxidation of injected carbon to CO. However, these improvements are achieved at the expense of creating a negative environmental impact due to the emission of CO.
The increased flow of injected carbon creates increased localized CO generation. Mixing of the CO with oxygen inside of the electric arc furnace is desirable but very difficult to arrange without excessive oxidation of the slag and electrodes. Although the single point of carbon injection provides localized heat release capable of increasing the temperature at a local cold spot and of improving furnace thermal efficiency, the rate of carbon injection is typically kept relatively low, due to the limited capability for dissipating the locally released heat and the limited ability of the locally generated CO stream to react with the additional oxygen stream creating environmentally acceptable CO.sub.2 prior to being exhausted out of the furnace.
The most modem electric arc furnaces are equipped with all or some of the above mentioned means for auxiliary heat input. Each auxiliary heat source plays its dedicated role to provide for additional heat input during a predetermined period of the steelmaking cycle and at the predetermined cold spot area affected by the positioning of each device.
With the development of the burner modifications capable of withstanding molten steel and slag splashing (U.S. Pat. No. 4,622,007; Re. 33,464), the use of multiple auxiliary burners positioned at the cold spots has become a common practice. These burners can also provide oxygen injection to cut pieces of heavy scrap located at the cold spots and to assist the slag foaming process.
A substantial increase in the use of oxygen for natural gas and carbon combustion in the electric arc furnace has resulted in reduced metallic yield due to excessive oxidation of scrap with injected oxygen. A portion of the FeO produced by the oxidation of scrap during the scrap melt down cycle is reduced back by reaction with carbon present in the slag. The reducing reaction of carbon and FeO is endothermic and therefore requires heat; thus, it can be effectively carded out only in hot slag containing particles of carbon which are well distributed in the slag. The reaction creates CO which bubbles through the slag forming a foamy slag layer.
Several known steelmaking methods that provide multiple point oxygen injection during the melt down cycle are based on the use of modified burners capable of high velocity oxygen injection following the burner firing cycle. (U.S. Pat. Nos. 4,622,007; 4,752,330). These methods use the burner flames to establish empty space in the scrap pile adjacent to the burner nozzle and to establish a pool of molten iron-carbon melt on the bottom of the furnace by the partial melting of scrap at the cold spots. After the empty space and pool of molten iron-carbon melt is established, the dedicated burner or burners initiate high velocity oxygen injection throughout the empty space toward the iron-carbon melt. The reaction of the injected oxygen with carbon in the melt or carbon in the slag results in rapid foamy slag formation. To establish the presence of solid carbon particles prior to the end of the scrap melt down cycle, the solid carbon may be previously charged into the furnace. When charged carbon reacts with injected oxygen, the foamy slag is rapidly formed in the areas affected by the injection of oxygen through the burners. Unfortunately, attempts to continually charge carbon prior to the end of the burner firing cycle by using known methods of carbon charging (with the scrap, through the hole in the furnace roof or through the slag door) have not been very successful because the charged carbon is quickly burned or carried out by the combustion products generated by the burners, creating a high level of CO emissions. On the other hand, earlier initiation of carbon injection through the slag door is not effective in creating a satisfactory foamy slag in other cold spot areas located far away from the slag door due to the presence of substantial amounts of unmolten scrap in the furnace. Unmolten scrap at the slag door blocks the penetration of injected carbon into the furnace.
Therefore, there is a need for a method and apparatus capable of generating foamy slag by combined carbon and oxygen injection during the early stage of the scrap melting cycle in the area or areas affected by heat input of the burners. This combined injection should be initiated after a substantial portion of scrap charged in this area or areas is molten and after an empty space suitable for carbon injection on the top of the iron-carbon melt is formed by the burners.
To provide for rapid and efficient melting of scrap, electric arc furnace burners utilize a highly concentrated oxidizing gas containing oxygen or a combination of oxygen and air. The excess oxygen, when introduced by the burners will react with scrap which has been heated by the burner flame.
Furthermore, it is considered advisable in many cases to introduce excess (i.e., above the stoichiometric ratio) amounts of oxygen through the burner to minimize incomplete combustion of fuel and to oxidize combustibles (e.g., oil, paint, plastics, etc.) charged with the scrap. Some known methods (U.S. Pat. No. 4,622,007, Re 33464) purposely use excessive oxygen to enhance hot scrap cutting to speed the scrap melting and to accelerate the rate at which the residual hot heavy scrap is submerged into the iron-carbon melt and, thus, to increase electric arc furnace throughput capacity and thermal efficiency. During the early stages of the melt down cycle, oxides charged with scrap and generated by the burners are mixed with the entire slag formed on the top of iron-carbon melt located near the burner locations. Therefore, it would be advantageous to inject small carbon particles into the slag layer near the burner locations and to provide heat to these spots to reduce iron oxides back to Fe. It would also be advantageous to use this of iron-oxide reduction with carbon to foam slag earlier during the melt down cycle when a substantial amount of scrap has not yet melted around the slag door and when foamy slag generated at the slag door area has not yet fully penetrated into the furnace to provide for good insulation of the electric arc.
The use of burners for melting scrap at the furnace slag door during the early part of the melt down cycle is necessary to establish an empty space and a hot environment prior to the initiation of combined oxygen/carbon injection for the purpose of forming foamy slag as early as possible. If oxygen and carbon are injected through the slag door too early or without burner assistance, the injected carbon cannot reach and/or react with the iron-carbon melt due to the presence of the cold scrap at the slag door. Under cold conditions, injected carbon primarily reacts with injected oxygen, forming CO, which then is exhausted from the furnace, generating a negative environmental impact instead of participating in the formation of foamy slag and in the reduction of FeO.
The firing of a movable burner at the slag door prior to the introduction of movable lance or lances into the furnace helps to melt scrap at the door, which allows more efficient use of carbon and oxygen injection by the door lances. However, rapid scrap melting at the slag door area also results in a very significant increase in the flow of ambient air infiltrating into the furnace. An increased volume of infiltrated air leads to an increased mass of nitrogen oxides (NO.sub.x) being generated inside the hot spots formed by the electric arc before foamy slag has been formed to submerge the arc and protect the extremely high temperature are region from contact with infiltrated air.
Therefore, there is a need for a method and apparatus capable of generating foamy slag in the electric arc furnace through the use of localized solid carbon and oxygen injection while simultaneously minimizing ambient air infiltration through the slag door.
The basic (as opposed to acidic) slag forming material(s) such as burnt lime, dolomitic lime, etc., are typically charged with the scrap or injected through an opening in the furnace wall. These materials are not well distributed and dissolved in the slag located at the areas affected by burners that have created empty space by melting a part of the scrap. To improve metallurgical characteristics of the slag being formed during the initial stage of scrap melting at the areas located near the burners, it is preferable to provide a method and apparatus for localized introduction of basic slag forming material at or near these areas. An empty space formed after the scrap is partially melted by the burners provides suitable conditions for localized injection of basic slag forming material, which improves foamy slag formation and permits earlier initiation of iron-carbon desulphurizing and dephosphorizing processes. Therefore, there is a need for a method and apparatus for localized introduction of basic slag forming material at the spots, assisted by the burners.
The oxidizing reactions between solid carbon and oxygen and/or solid carbon and molten oxides generates CO which is partially oxidized to CO.sub.2 when mixed with oxygen at high temperature conditions inside the furnace. When this reaction occurs under conditions permitting the heat released by the post-combustion of CO to be efficiently transferred to the scrap to be melted or to the iron-carbon melt, the furnace throughput capacity and thermal efficiency is increased. Therefore, the need exists to provide for localized post-combustion of CO with an oxidizing gas which is introduced in areas where the CO concentration is substantially higher than the average CO concentration in the furnace exhaust gases.
During the EAF operation, a substantial volume of the slag is accumulated on the side walls providing the insulating layer that protects the wall surface from being overheated by the arc. Keeping slag on the side walls is especially beneficial for the panels located at the hot spots of the furnace. This desirable build-up of slag on the furnace side walls makes it necessary to use movable burners and devices for oxygen and solid material injection that are designed to operate through the open slag door and/or through the openings located in the EAF roof or in the top part of the side panel. The location of these openings avoids the problem of openings plugging with the slag, since only a limited volume of slag is splashed in the vicinity of these locations. When movable burners or lances are used, they are located in areas visible from the operating room so that the furnace operator can observe the movement of the devices. Unfortunately, this limits the use of the devices and increase the cost of the installation. During the last several years, new burners have been introduced that can be permanently installed in the lower part of the side panel and near the slag line, and that are capable of protecting themselves from plugging with slag. This has significantly improved the performance of the burners and led to an increase in the number of burners utilized in the furnaces. The presence of multiple burners located in multiple points of the EAF side walls close to the slag layers and iron-carbon melt can potentially be used to expand the burner functions by making them responsible for carbonaceous fuel injection combined with oxygen injection for foamy slag formation, iron-carbon melt refining, and CO post-combustion purposes.
Therefore, there is a need for a method and apparatus for combined oxygen and carbonaceous fuel injection that can be permanently installed in the water-cooled panel of the EAF, preferably close to the slag line, and for such an apparatus that can operate without movement and without the use of expensive moving mechanisms.
The increased utilization of solid carbonaceous fuel and oxygen in electric arc furnaces and the use of steel scrap containing plastic, paint, oil and other carbon beating materials has led to an increase in carbon monoxide and hazardous hydrocarbon generated during the scrap melting cycle of the electric are steelmaking process. At the same time, in order to produce high quality steel and to minimize metallic impurities inputted by scrap, electric arc furnace shops have increased utilization of solid pig iron, iron scrap, direct reduced iron, iron carbide and other ferrous materials having high carbon content. After these ferrous materials are melted down, the melt is refined to oxidize carbon and other impurities in the iron-carbon melt. This oxidation of carbon generates hot CO emission from the molten bath.
Modem electric arc furnaces are equipped with a means to post-combust CO in the furnace exhaust gases by the use of ambient air inspirated through the break-flange connecting the electric arc furnace with the air pollution control system and by the use of a combustion chamber located downstream of the electric arc furnace. This combustion chamber is designed to provide for the additional residence time and the mixing needed for the reaction of CO with ambient air which has been inspirated upstream of the combustion chamber but down stream of the furnace break-flange.
To reduce the levels of CO and hazardous hydrocarbon to environmentally desired standards, the post-combustion of CO should be performed within and downstream of the electric arc furnace. Post-combustion of CO in the electric arc furnace is most beneficial when the heat released by oxidation of CO to CO.sub.2 is efficiently transferred to the scrap and to the melt. This results in the reduction of electrical energy consumption and/or in an increase in the furnace production rate.
During the initial, cold phase of the scrap melting cycle, the scrap pile located at the slag door blocks ambient air infiltration into the furnace and therefore into the areas where the heat is inputted by the arc and by the auxiliary burners (when burners are used to add auxiliary heat to the scrap melting process). The lack of oxygen in high temperature spots where initial CO and hazardous hydrocarbons are formed (by volatizing and incomplete combustion of charged hydrocarbons and carbon containing materials) prevents oxidation to CO.sub.2 of the hot CO generated in these spots. This formed CO is exhausted away from the furnace by the suction created at the break-flange of the exhaust elbow of the furnace by exhaust fan of the air pollution control system. CO, unburned hydrocarbons and oxygen contained in infiltrated air mix and partially react in the narrow conduit formed by the exhaust elbow located downstream of the furnace and upstream of the break-flange. However, due to the low temperature of exhaust gases during the initial cold phase of scrap melting and very short retention time, substantial quantities of CO and unburned hydrocarbons survive exhaust elbow mixing and arrive at the break-flange of the electric arc furnace. The break-flange is used to connect the furnace exhaust elbow and the exhaust duct and comprises an inspirating gap which provides for inspiration of additional secondary ambient air into the exhaust gases evacuated from the furnace. When this additional inspirated air is mixed with cold exhaust gases during the initial cold phase of scrap melting, this mixing results in further reduction of the exhaust gas temperature. This colder exhaust then travels into the combustion chamber which provides for additional mixing and retention time. Unfortunately, this additional retention time can not ensure completion of CO post-combustion in the low temperature exhaust gases prior to flue gas emission into the atmosphere. Therefore, it is desirable to raise the temperature of the exhaust gases reaching the combustion chamber by reducing the inspirating gap during the initial stage of scrap melting and/or by the firing of additional burners into the combustion chamber to raise the flue gas temperature to insure ignition of the CO and unburned hydrocarbons.
During the hot phase of melt refining, a substantial amount of CO is emitted from the bath. A substantial amount of CO is also emitted from the carbon containing slag, especially when foamy slag practice is used in the electric arc furnace utilizing solid carbonaceous particles injection. During these periods of increased CO emissions, it is advisable to maximize the amount of ambient air inspirated at the break flange. However, this maximum amount of ambient air can periodically be insufficient to provide adequate oxygen to complete CO oxidation during peaks of CO emissions. Therefore, the need exists to provide for additional injection of oxidizing gas into the furnace and/or into the combustion chamber downstream of the electric arc furnace to post-combust CO generated during the hot phase of the steelmaking process.
The known methods of electric arc furnace steelmaking use the slag door to introduce multiple movable lances for the injection into the process of oxygen and solid carbonaceous particles as well as for the introduction of a burner flame to melt the scrap near the slag door. Also known is the injection and/or batch charging of basic slag forming materials and slag enhancers through the slag door and the injection of additional oxygen via a movable oxygen injecting lance through the slag door for the purpose of CO post-combustion. All the above technologies use multiple manual and/or automated movable lances that are moved into the furnace and removed during the steelmaking cycle.
When ambient air infiltrated in the furnace passes through the hot spots located near the arc or mixes with the high temperature flames generated by oxy-fuel burners, nitrogen and oxygen of the air react under such conditions to form nitrogen oxides (NO.sub.x), comprised primarily of NO. NO further partially reacts with CO, volatized hydrocarbons and soot particles so that the total amount of NO is reduced before the exhaust gases leave the electric arc furnace. When the exhaust gases pass through the combustion chamber during the hot phase of the steelmaking cycle, the reaction between nitrogen and oxygen can be triggered inside of the hot spots created by the hot flames or by streams of highly concentrated oxidizing gas injected for the purpose of CO post-combustion located inside the combustion chamber interior. This may increase NO.sub.x emission from electric arc facilities. Therefore, there is a need to minimize NO emission from electric arc furnaces while utilizing high temperature times and/or oxygen injection in the furnace and in the furnace combustion chamber.
The slag door of the furnace is the most accessible opening existing in the electric arc furnace for the introduction of movable lance(s) and burning means. Known apparatuses for carbon injection, lime injection and oxygen injection comprise multiple, separately movable water-cooled lances and/or consumable pipe-lances that are submerged into the melt. The burner(s) and the multiple lances are introduced utilizing multiple movement mechanisms through the slag door during different periods of the steelmaking cycle to provide for various steelmaking process inputs. The use of multiple lances and/or burner(s) makes it difficult and expensive to carry out automatic and simultaneous introduction of the time, oxygen, carbon and basic slag forming material through the slag door.
Therefore, there is a need for combining the burner means, a carbon injection means, an oxygen injection means and optionally, a basic slag forming material injection means integrated via movable water-cooled lances capable of efficiently operating through the slag door during the entire steelmaking process.
When lumps of basic slag forming material, such as burnt lime, dolomite lime, raw dolomite, lime stone, etc., are charged, these materials should be well distributed inside of the furnace. Good distribution of lump carbonaceous fuel such as anthracite, coke, etc., is also important to ensure good performance of charged materials. Unfortunately, currently available apparatuses are not able to accomplish the efficient introduction and good distribution of the materials during the desired period of the steelmaking cycle. This results in an incremental use of basic slag forming material, a correspondingly increase in electrical energy usage due to the additional energy required to melt the incremental basic slag forming material, and increased CO emissions due to oxidation of the incremental solid carbonaceous fuel material. This also results in incremental use of solid carbonaceous materials which are utilized with low efficiency, but which provide additional sulfur and nitrogen input to the slag and, therefore, to the melt.
Therefore, there is a need for a method and apparatus for the introduction of lump carbonaceous fuel and basic slag forming materials and for allowing good distribution of the materials inside of the slag, thereby reducing their consumption, the sulfur and nitrogen content of the melt and the mount of electrical energy per ton of steel produced.
The continuous increase in electric are steelmaking process productivity has been achieved in modeen furnaces by the use of more powerful arcs and by the use of additional heat sources. This increase has led to the reduction in tap-to-tap time and therefore, the time period available for the use of burners and movable slag door lances.
Therefore, there is also a need for a method and apparatus allowing for rapid scrap melting near the slag door area to provide for the earlier introduction of movable injecting means into the furnace through the slag door.